This invention relates in general to an optical system for reading a video disc. More particularly, the invention concerns an improved optical system for deriving a control signal to maintain tracking registration of the reading beam in a reflective mode video disc playback apparatus.
In the field of video information storage and retrieval the video disc record as been proposed as an adjunct to the conventional home color television receiver in order to augment the utility of such a receiver by employing it as a player for recorded video and audio program materials. The program is stored in a medium, such as a vinyl disc, somewhat similar to the familiar audio disc. A record of the type herein considered has the video or other information stored in a spiral track which track physically can take the form of a train of pits and lands. Such a track is intended to by read by a beam of light, preferably a laser beam. The stored program material may include luminance, chroma, audio and synchronizing signals judiciously positioned in segments of the frequency spectrum deemed to be convenient for recording on a disc but, at the same time, subject to transformation to a frequency distribution that is compatible with the type of signal (NTSC) for which the receiver was designed to process. As such, the information stored in the record track preferably features double interlaced fields for each picture frame, with each convolution of the spiral containing two such fields together with requisite synchronizing information.
As noted above, the video information may be stored in a track comprising a succession of pits and lands; on the other hand, the information can be stored in a mechanically generated undulating groove, of a type similar to that employed in conventional audio recordings. In any event, while the invention is equally usable with either type of recording, there appears to be greater promise in the pit and land type of track at the present; accordingly, the invention will henceforth be described in connection with such a track.
In a preferred recording technique the information to be stored is embodied in a baseband signal which is used to frequency modulate a carrier. The modulated carrier is then employed to modulate a laser writing beam that creates, in conjunction with photolithographic techniques, a master recording comprising a spiral track formed of a succession of pits and lands of uniform widths but varying lengths. In a disc record reproduced from such a master, the pits and lands of the track will likewise have uniform widths and also lengths that are variable along the direction of the track so that these pits and lands constitute a spatial representation of the temporal variations of the frequency modulated carrier signal.
The stored information is retrieved by scanning the track with a laser reading beam and utilizing a photoreceptor to respond to the light energy transmitted through or reflected from the track. This light energy is diffracted or modulated in accordance with the information stored in the track. In the situation where the video disc is transmissive to the reading beam, the photoreceptor is located along the beam path and in the far field pattern of the diffracted beam on the opposite side of the record scanned by the beam. Positioned in this manner the photoreceptor collects a portion of the light transmitted through the disc. On the other hand, where the disc is interrogated in a reflective mode, the photoreceptor is located on the same side of the track as the reading beam, again in the far field pattern, to enable it to respond to diffracted light reflected from the track. In either case, the pits and lands serve to diffract or scatter the light of the reading beam so that the photoreceptor, in response to these variations in light intensity, develops an electrical signal bearing modulation which is in accordance with the stored information represented by the pits and lands.
In practice, optical video systems of the type herein considered frequently experience radial tracking erros due to eccentricity of the disc and/or the track which results in mis-registration of the reading beam and the record track. Additionally, time base errors are also encountered when the disc or track is afflicted by eccentricity. These tracking and timing errors are compensated by means of correction apparatus which can take the form of the electro-optical tracking arrangement disclosed and claimed in application Ser. No. 456,918, now U.S. Pat. No. 3,946,166, filed Apr. 1, 1974, in the name of Karl H. Wossidlo and assigned to the assignee of the present invention, now U.S. Pat. No. 3,946,166 which issued Mar. 23, 1976. The aforesaid arrangement includes a mirror which is inserted in the path of the reading beam and which is supported for adjustable displacement about a pair of orthogonally disposed axes coincident with a radial and a tangent to the track at the point where the track is being interrogated. Respective correction signals, indicative of the sense and extent of radial tracking and time base errors, are derived and applied to assigned drivers which serve to tilt the mirror about its orthogonal axes to restore proper tracking and timing.
In the case of radial tracking, an asymmetry in the far field pattern of the diffracted read beam is detectable when the read beam departs from registration with the track. This asymmetry is monitored by a pair of photodiodes which derive a tracking error signal. A particularly attractive photodiode pair arrangement for deriving a radial tracking correction signal is described in copending application Ser. No. 439,680, now U.S. Pat. No. 3,931,459, filed Feb. 4, 1974 in the name of Adrianus Korpel, which application is also assigned to the same assignee as the present invention and issued as U.S. Pat. No. 3,931,459 on Jan. 6, 1976.
Experience has revealed, however, that prior art tracking and time base correction arrangements suffer the undesirable shortcoming of causing a transverse motion of the far field pattern across the photoreceptor during excursions of the tracking mirror. If this motion of the far field pattern produces a positive spurious feedback, then the tracking servo system will be effectively disabled. It is appreciated, however, that some motion of the far field pattern may be unavoidable; in that case, the movement of the pattern should be restricted to a sense which leads to a negative feedback within the tracking servo loop.